If I schedule a particular App, will that App be able to be used when scheduled screen time is up?

The scheduled screen time applies to the entire device and is the "master" of all schedules:

Example:

Scheduled Screen Time set from 9am to 7pm.

Clock App scheduled from 7am to 10pm

Outcome = Clock App will still only work from 9am to 7pm.

What if you want Clock App to work outside the 9am to 7pm? Then you would add the Clock App to the Always Allowed list and check the "Anytime" option so that it works 24/7 and does not adhere to the scheduled screen time. To do so, Login into the Boomerang Web Management Dashboard, click on the device and then the Always Allowed Tab.